A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103, which in turn vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. The cochlea 104 includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The scala tympani forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid filled cochlea 104 functions as a transducer to generate electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain. Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104.
In some cases, hearing impairment can be addressed by a cochlear implant that electrically stimulates auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along an implant electrode. FIG. 1 shows some components of a typical cochlear implant system where an external microphone provides an audio signal input to an external signal processor 111 which implements one of various known signal processing schemes. For example, signal processing approaches that are well-known in the field of cochlear implants include continuous interleaved sampling (CIS) digital signal processing, channel specific sampling sequences (CSSS) digital signal processing (as described in U.S. Pat. No. 6,348,070, incorporated herein by reference), spectral peak (SPEAK) digital signal processing, and compressed analog (CA) signal processing. The processed signal is converted by the external signal processor 111 into a digital data format, such as a sequence of data frames, for transmission into a receiving stimulator processor 108. Besides extracting the audio information, the receiver processor in the stimulator processor 108 may perform additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through electrode lead 109 to an implanted electrode array 110. Typically, the electrode array 110 includes multiple stimulation contacts on its surface that provide selective electrical stimulation of the cochlea 104.
Besides getting the processed audio information to the implanted stimulator processor 108, existing cochlear implant systems also need to deliver electrical power from outside the body through the skin to satisfy the power requirements of the implanted portion of the system. FIG. 1 shows an arrangement based on inductive coupling through the skin to transfer both the required electrical power and the processed audio information. As shown in FIG. 1, a primary transmitter coil 107 (coupled to the external signal processor 111) is externally placed on the patient's skin adjacent to a subcutaneous secondary receiving coil in the stimulator processor 108. This arrangement inductively couples a radio frequency (rf) electrical signal to the stimulator processor 108 which is able to extract both a power component from the rf signal it receives, and the audio information for the implanted portion of the system.
The power component is extracted and stored in one or more (rechargeable) batteries, for example, in the stimulator processor 108. The current consumption supplied from the implanted batteries to the implanted components is an important system factor. In relative terms, the amount of current drawn from the batteries by the implanted components is very high and contributes to the end of battery lifetime. In contrast, low current consumption increases the number of (de)charging cycles which directly increases battery lifetime. Especially for fully implantable cochlear implants, low current consumption is important because the end of battery lifetime requires a surgical operation that bears many of the risks known with CI implantation.